When an ion produced from a compound in a sample is made to move in a gas medium (or liquid medium) by the effect of an electric field, the ion moves at a speed proportional to the mobility which is determined by the strength of the electric field, size of the ion and other factors. Ion mobility spectrometry is a measurement method utilizing this ion mobility for an analysis of a compound. Analyzing devices employing this measurement method are generally called the “ion mobility spectrometer”, “ion mobility meter” or otherwise. In the following description, an ion mobility spectrometer is referred to as an “IMS device”.
Commonly used IMS devices include an ion source for ionizing compound molecules in a sample, a drift region formed within a housing having a cylindrical form (or other appropriate forms) for separating ions according to their ion mobility, and a detector for detecting the ions which have travelled through the drift region (for example, see Patent Literature 1). Normally, a uniform electric field which exhibits a downward potential gradient in the direction in which the ion travels (ion-moving direction), i.e. which has the effect of accelerating the ions, is formed within the drift region. Additionally, a stream of neutral gas (which is normally an inert gas) is formed in the opposite direction to the accelerating direction by the electric field, i.e. the ion-moving direction.
The ions produced in the ion source and introduced into the drift region travel along the downward potential gradient while colliding with the neutral gas flowing in the opposite direction. During this movement, the ions are temporally separated according to their ion mobility which depends on the size, three-dimensional structure, electric charge and other properties of the ions. Ions having different ion mobilities reach the detector having certain intervals of time. If the electric field within the drift region is uniform, it is possible to calculate the collision cross-section between an ion and the neutral gas based on the drift time required for the ion to pass through the drift region.
In the case of analyzing a compound in a gas sample using an IMS device, an ion source which ionizes the compound using beta rays emitted from a radioactive isotope, such as 63Ni, or an atmospheric pressure ion source which uses corona discharge, or other types of ion sources are commonly used (see Patent Literatures 1 and 2). Such an IMS device can be used as a detector for a gas chromatograph (GC). A GC-IMS in which an IMS device is connected to the exit port of the column of a GC has been practically used. However, the range of substances that can be detected with GC-IMS is limited to volatile substances that can be vaporized in the sample injection section of the GC. Accordingly, in order to enable the detection of a wider range of substances inclusive of hard-to-volatile and non-volatile substances, an LC-IMS which uses an IMS device as the detector for a liquid chromatograph (LC) has been developed.
In the LC-IMS, it is necessary to produce gas-phase ions from a compound in a liquid sample in the ion source of the IMS device. For this purpose, an ion source which employs atmospheric pressure ionization is used, such as the atmospheric pressure chemical ionization (APCI), electrospray ionization (ESI) or atmospheric pressure photoionization (APPI), all of which are also commonly used in liquid chromatograph mass spectrometers (LS-MS).